Clarke has found a winning system with a big target man up front. We've got absolutely no-one to replace Dykes when he's out so Clarke has a choice of changing the entire system to fit the likes of Griffiths in, or trying to find another target man. He's obviously hoping McBurnie would be that guy, however Clarke will be well aware that it's not really working.
With international games few and far between, persevering with McBurnie or trying to find a whole new system is a difficult balance to strike. I'm sure he'll make the right judgement either way.